June 2011
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comparing perception and introspection

Naber (see citation) and others have carefully studied visual rivalry (those stimuli that seem to flip between two distinct images). Rivalry has been billed as having a sharp change between the two images. Is the sharpness part of the perception or is it a function of our awareness? Their results show that the changes are more gradual then previously thought. The changes in our awareness are sharper than in our unconscious perception. Reporting the conscious change with a key or button make it appear even sharper.


Here is the abstract:

Rivalry is a common tool to probe visual awareness: a constant physical stimulus evokes multiple, distinct perceptual interpretations (‘‘percepts’’) that alternate over time. Percepts are typically described as mutually exclusive, suggesting that a discrete (all-or-none) process underlies changes in visual awareness. Here we follow two strategies to address whether rivalry is an all-or-none process: first, we introduce two reflexes as objective measures of rivalry, pupil dilation and optokinetic nystagmus (OKN); second, we use a continuous input device (analog joystick) to allow observers a gradual subjective report. We find that the ‘‘reflexes’’ reflect the percept rather than the physical stimulus. Both reflexes show a gradual dependence on the time relative to perceptual transitions. Similarly, observers’ joystick deflections, which are highly correlated with the reflex measures, indicate gradual transitions. Physically simulating wave-like transitions between percepts suggest piece-meal rivalry (i.e., different regions of space belonging to distinct percepts) as one possible explanation for the gradual transitions. Furthermore, the reflexes show that dominance durations depend on whether or not the percept is actively reported. In addition, reflexes respond to transitions with shorter latencies than the subjective report and show an abundance of short dominance durations. This failure to report fast changes in dominance may result from limited access of introspection to rivalry dynamics. In sum, reflexes reveal that rivalry is a gradual process, rivalry’s dynamics is modulated by the required action (response mode), and that rapid transitions in perceptual dominance can slip away from awareness.


(In case ‘optokinetic nystagmus’ is a term you have not met – it is the type of eye movement we use to track a movement, when we move our eyes with the movement and then flick back to the starting point and track the movement again. The name was new to me. They had a right moving grating to one eye and a left moving one to the other in one of their experiments and could tell which was perceived by the direction of the returning flick of the eyes.)


And here is their final conclusion:

Reflexes reveal that rivalry is a gradual process, its dynamics are affected by the response mode, and fast changes in dominance can slip away unnoticed (or unreported) by observers. Consequently, reflexes allow access to earlier (subconscious) levels of perception, which are unavailable to awareness, and thus stress the limits of relying on introspection alone.


In some models of conscious awareness, there are time slices of measurable duration where events do not rise to consciousness (like the space between frames of a movie). This, to my mind, could be part of the reason for the sharpness of change and the loss of very short events, in our awareness as opposite to our unconscious perception.

Naber, M., Frässle, S., & Einhäuser, W. (2011). Perceptual Rivalry: Reflexes Reveal the Gradual Nature of Visual Awareness PLoS ONE, 6 (6) DOI: 10.1371/journal.pone.0020910

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